Multiple reactant nozzles for a flowing reactor

ABSTRACT

A collection of zinc oxide nanoparticles have been produced by laser pyrolysis. The zinc oxide nanoparticles have average particle diameters of less than about 95 nm and a very narrow particle size distribution. The laser pyrolysis process is characterized by the production of a reactant stream within the reaction chamber, where the reactant stream includes a zinc precursor and other reactants. The zinc precursor can be delivered as an aerosol.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 08/962,362, filed on Oct. 31, 1997, incorporatedherein by reference.

BACKGROUND OF THE INVENTION

[0002] The invention relates to zinc oxide nanoparticles and methods offorming zinc oxide nanoparticles.

[0003] Advances in a variety of fields have created a demand for manytypes of new materials. In particular, a variety of chemical powders canbe used in many different processing contexts. Specifically, chemicalpowders can be used in the production of electronic devices, such asresistors and the like.

[0004] In addition, electronic displays often use phosphor material,which emit visible light in response to interaction with electrons.Phosphor materials can be applied to substrates to produce cathode raytubes, flat panel displays and the like. Improvements in display devicesplace stringent demands on the phosphor materials, for example, due todecreases in electron velocity and increases in display resolution.Electron velocity is reduced in order to reduce power demands. Inparticular, flat panel displays generally require phosphors responsiveto low velocity electrons.

[0005] In addition, a desire for color display requires the use ofmaterials or combination of materials that emit light at differentwavelengths at positions in the display that can be selectively excited.A variety of materials have been used as phosphors. In particular, zincoxide powders have found use as phosphors, either alone or incombination with other materials. In order to obtain materials that emitat desired wavelengths of light, activators have been doped intophosphor material. Alternatively, multiple phosphors can be mixed toobtain the desired emission. Furthermore, the phosphor materials mustshow sufficient luminescence.

SUMMARY OF THE INVENTION

[0006] In a first aspect, the invention pertains to a collection ofparticles comprising crystalline zinc oxide, the collection of particleshaving an average diameter less than about 95 nm. The zinc oxideparticles can be incorporated into a variety of devices includingelectronic devices, such as an electrical resistor.

[0007] In another aspect, the invention pertains to a method forproducing zinc oxide particles. The method includes pyrolyzing areactant stream comprising a zinc precursor and an oxygen source in areaction chamber, where the pyrolysis is driven by heat absorbed from alight beam.

[0008] In a further aspect, the invention pertains to a method forproducing zinc oxide particles, the method including pyrolyzing areactant stream comprising a zinc precursor aerosol in a reactionchamber, where the pyrolysis is driven by heat absorbed from a lightbeam.

[0009] Moreover, the invention pertains to a reaction system comprising:

[0010] a reaction chamber having an outlet along a reactant path;

[0011] a reactant delivery apparatus that combines reactants within thereaction chamber from a plurality of reactant inlets, such that thecombined reactants are directed along the reactant path; and

[0012] a light source that directs a light beam at the combinedreactants along the reactant path.

[0013] In some embodiments of the reaction system with a plurality ofreactant inlets, the reactant delivery apparatus includes:

[0014] an aerosol delivery apparatus that produces an aerosol along thereactant path; and

[0015] a gaseous reactant delivery apparatus that combines a gaseousreactant with the aerosol along the reactant path within the reactionchamber.

[0016] The aerosol delivery apparatus can include a conduit connected toa gas supply.

[0017] In alternative embodiments of the reaction system, reactantdelivery apparatus includes two gas ports oriented to combine twogaseous reactants along the reactant path within the reaction chamber.In other alternative embodiments, the reactant delivery apparatusincludes two aerosol delivery apparatuses oriented to combine twoaerosol reactants along the reactant path within the reaction chamber.

[0018] The reaction system can further include a shielding gas portoriented to direct a shielding gas to limit the spread of the combinedreactants along the reactant path. The reactant system can have areactant delivery apparatus that combines three or more reactants withinthe reaction chamber along the reactant path from three or more reactantinlets.

[0019] In another aspect, the invention features a method of producingchemical powders, the method comprising:

[0020] combining two reactants within a reaction chamber from aplurality of reactant inlets, such that the combined reactants aredirected along a reactant path; and

[0021] pyrolyzing the reactants flowing along the reaction path with anintense light source.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a schematic sectional view of a solid precursor deliverysystem taken through the center of the system.

[0023]FIG. 2 is a schematic, sectional view of an embodiment of a laserpyrolysis apparatus taken through the middle of the laser radiationpath. The lower inserts are bottom views of the injection nozzle withone or two reactant inlets, and the upper insert is a bottom view of thecollection nozzle.

[0024]FIG. 3 is a schematic view of a reactant delivery apparatus forthe delivery of vapor reactants to the laser pyrolysis apparatus of FIG.2.

[0025]FIG. 4 is schematic, side view of a reactant delivery apparatusfor the delivery of an aerosol reactant to the laser pyrolysis apparatusof FIG. 2.

[0026]FIG. 5 is schematic, side view of an alternative embodiment of areactant delivery apparatus for the delivery of two aerosols to thelaser pyrolysis apparatus of FIG. 2.

[0027]FIG. 6 is a schematic, perspective view of a reaction chamber ofan alternative embodiment of the laser pyrolysis apparatus, where thematerials of the chamber are depicted as transparent to reveal theinterior of the apparatus.

[0028]FIG. 7 is a perspective view of an embodiment of a laser pyrolysisapparatus with an elongated reaction chamber.

[0029]FIG. 8 is a sectional view of the laser pyrolysis apparatus ofFIG. 7, where the section is taken along line 8-8 of FIG. 7.

[0030]FIG. 9 is a schematic, sectional view of an apparatus for heattreating nanoparticles, in which the section is taken through the centerof the apparatus.

[0031]FIG. 10 is a sectional view of an embodiment of a display deviceincorporating a phosphor layer.

[0032]FIG. 11 is a sectional view of an embodiment of a liquid crystaldisplay incorporating a phosphor for illumination.

[0033]FIG. 12 is a sectional view of an electroluminescent display.

[0034]FIG. 13 is a sectional view of an embodiment of a flat paneldisplay incorporating field emission display devices.

[0035]FIG. 14 is a schematic, sectional view of a resistor device takenthrough the center of the device.

[0036]FIG. 15 is an x-ray diffractogram of zinc oxide nanoparticlesproduced by laser pyrolysis.

[0037]FIG. 16 is a TEM micrograph of nanoparticles whose x-raydiffractogram is shown of FIG. 15.

[0038]FIG. 17 is a plot of the distribution of primary particlediameters for the nanoparticles shown in the TEM micrograph of FIG. 16.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] Small scale particles can be used as improved phosphor particles.In particular, particles on the order of 100 nm or less have superiorprocessing properties to produce displays, and they have goodluminescence. Significantly, the band gap of these materials is sizedependent at diameters on the order of 100 nm or less. Therefore,particles with a selected, narrow distribution of diameters can serve asa phosphor at one color (wavelength) while particles of the same ordifferent material with similarly selected average diameter and narrowdistribution of sizes can serve as a phosphor at a different color. Inaddition, the small size of the particles can be advantageous for theproduction of higher resolution displays.

[0040] Appropriate particles generally are chalcogenides, especiallyZnO, ZnS, TiO₂, and Y₂O₃. Preferred particles have a desired emissionfrequency and are highly luminescent. In addition, preferred particleshave persistent emission, i.e., the emission decays over a significantperiod of time following electrical stimulation of the material.Specifically, there should be sufficient persistence of the emission toallow for human perception. Suitable particles generally aresemiconductors, and their emission frequency is determined by the bandgap. Preferably, the luminescing state has an energy such that littleenergy is wasted as heat.

[0041] Laser pyrolysis, as described below, is an excellent way ofefficiently producing ZnO, ZnS, TiO₂, and Y₂O₃ particles with narrowdistributions of average particle diameters, Generally, the particlesize distributions drop off very rapidly such that it does not have asignificant tail. A basic feature of successful application of laserpyrolysis for the production of appropriate small scale particles isgeneration of a reactant stream containing a metal precursor compound, aradiation absorber and a reactant serving as an oxygen or sulfur source,as appropriate. The metal precursor can act as the radiation absorbingcompound and/or the oxygen source. The reactant stream is pyrolyzed byan intense laser beam. The intense heat resulting from the absorption ofthe laser radiation induces the reaction of the metal compound precursorin the oxygen or sulfur environment. As the reactant stream leaves thelight beam, the particles are rapidly quenched.

[0042] The reactants for performing laser pyrolysis can be supplied invapor form. Alternatively, one or more reactants can be supplied as anaerosol. The use of an aerosol provides for the use of a wider range ofmetal precursors than are suitable for vapor delivery only. Thus, lessexpensive precursors can be used with aerosol delivery in some cases.Also, aerosol delivery can be used for high particle production rates.Suitable control of the reaction conditions with the aerosol results innanoscale particles with a narrow particle size distribution.

[0043] In particular, laser pyrolysis is a convenient approach for theformation of zinc oxide nanoparticles. The crystalline zinc oxidenanoparticles have very small particle diameters and a very narrowparticle size distribution. Zinc oxide powders are useful as phosphorsand can be used in a variety of other applications, such as theproduction of electronic components, for example, resistors of varioustypes.

[0044] A. Particle Production Using Laser Pyrolysis

[0045] Laser pyrolysis has been discovered to be a valuable tool for theproduction of nanoscale phosphorescent particles, including, inparticular, zinc oxide particles. In addition, the particles produced bylaser pyrolysis are a convenient material for further processing toexpand the pathways for the production of desirable zinc oxideparticles. Thus, using laser pyrolysis alone or in combination withadditional processes, a wide variety of zinc oxide particles can beproduced.

[0046] The reaction conditions determine the qualities of the particlesproduced by laser pyrolysis. The reaction conditions for laser pyrolysiscan be controlled relatively precisely in order to produce particleswith desired properties. The appropriate reaction conditions to producea certain type of particles generally depend on the design of theparticular apparatus. Specific conditions used to produce zinc oxideparticles in a particular apparatus are described below in the Example.Furthermore, some general observations on the relationship betweenreaction conditions and the resulting particles can be made.

[0047] Increasing the laser power results in increased reactiontemperatures in the reaction region as well as a faster quenching rate.A rapid quenching rate tends to favor production of high energy phases,which may not be obtained with processes near thermal equilibrium.Similarly, increasing the chamber pressure also tends to favor theproduction of higher energy structures. Also, increasing theconcentration of the reactant serving as the oxygen source in thereactant stream favors the production of particles with increasedamounts of oxygen.

[0048] Reactant flow rate and velocity of the reactant gas stream areinversely related to particle size so that increasing the reactant gasflow rate or velocity tends to result in smaller particle sizes. Also,the growth dynamics of the particles have a significant influence on thesize of the resulting particles. In other words, different forms of aproduct compound have a tendency to form different size particles fromother phases under relatively similar conditions. Light intensity/laserpower also influences particle size with increased light intensityfavoring larger particle formation for lower melting materials andsmaller particle formation for higher melting materials.

[0049] Laser pyrolysis has been performed generally with gas phasereactants. The use of exclusively gas phase reactants is somewhatlimiting with respect to the types of precursor compounds that can beused practically. Thus, techniques have been developed to introduceaerosols containing reactant precursors into laser pyrolysis chambers.Improved aerosol delivery apparatuses for reactant systems are describedfurther in commonly assigned and copending U.S. patent application Ser.No. 09/188,670 to Gardner et al., entitled “Reactant DeliveryApparatuses,” filed Nov. 9, 1998, incorporated herein by reference.

[0050] Using aerosol delivery apparatuses, solid precursor compounds canbe delivered by dissolving the compounds in a solvent. Alternatively,powdered precursor compounds can be dispersed in a liquid/solvent foraerosol delivery. Liquid precursor compounds can be delivered as anaerosol from a neat liquid, a multiple liquid dispersion or a liquidsolution, if desired. Aerosol reactants can be used to obtain asignificant reactant throughput. The solvent, if any, can be selected toachieve desired properties of the solution. Suitable solvents includewater, methanol, ethanol, isopropyl alcohol, other organic solvents andmixtures thereof. The solvent should have a desired level of purity suchthat the resulting particles have a desired purity level. Some solvents,such as isopropyl alcohol, are significant absorbers of infrared lightfrom a CO₂ laser such that no additional laser absorbing compound isneeded within the reactant stream.

[0051] If aerosol precursors are formed with a solvent present, thesolvent preferably is rapidly evaporated by the light beam in thereaction chamber such that a gas phase reaction can take place. Thus,the fundamental features of the laser pyrolysis reaction are unchanged.Nevertheless, the reaction conditions are affected by the presence ofthe aerosol. Below in the Example, conditions are described for theproduction of zinc oxide nanoparticles using aerosol precursors in aparticular laser pyrolysis reaction chamber. Thus, the parametersassociated with aerosol reactant delivery can be explored further basedon the description below.

[0052] A number of suitable solid, zinc precursor compounds can bedelivered as an aerosol from solution. For example, zinc chloride(ZnCl₂) and zinc nitrate (Zn(NO₃)₂) are soluble in water and someorganic solvents, such as isopropyl alcohol. The compounds are dissolvedin a solution preferably with a concentration greater than about 0.5molar. Generally, the greater the concentration of precursor in thesolution the greater the throughput of reactant through the reactionchamber. As the concentration increases, however, the solution canbecome more viscous such that the aerosol has droplets with larger sizesthan desired. Thus, selection of solution concentration can involve abalance of factors in the selection of a preferred solutionconcentration.

[0053] Appropriate zinc precursor compounds for gaseous deliverygenerally include zinc compounds with reasonable vapor pressures, i.e.,vapor pressures sufficient to get desired amounts of precursor vapor inthe reactant stream. The vessel holding liquid or solid precursorcompounds can be heated to increase the vapor pressure of the zincprecursor, if desired. Suitable solid, zinc precursors with sufficientvapor pressure of gaseous delivery include, for example, zinc chloride(ZnCl₂). A suitable container for heating and delivering a solidprecursor to a laser pyrolysis apparatus is shown in FIG. 1. Suitableliquid zinc precursor compounds include, for example, diethyl zinc(Zn(C₂H₅)₂) and dimethyl zinc (Zn(CH₃)₂).

[0054] Referring to FIG. 1, the solid precursor delivery apparatus 50for vapor delivery includes a container 52 and a lid 54. A gasket 56 islocated between container 52 and lid 54. In one preferred embodiment,container 52 and lid 54 are made from stainless steel, and gasket 56 ismade from copper. In this embodiment, lid 54 and gasket 56 are bolted tocontainer 52. Other inert materials, such as Pyrex®, suitable for thetemperatures and pressures applied to the solid precursor system can beused. Container 52 is surrounded with a band heater 58, which is used toset the temperature of the delivery apparatus 50 at desired values.Suitable band heaters are available from Omega Engineering Inc.Stamford, Conn. The temperature of the band heater can be adjusted toyield a desired vapor pressure of the precursor compound. Additionalportions of the precursor delivery system can be heated to maintain theprecursor in a vapor state after it has left container 52.

[0055] Preferably, a thermocouple 60 is inserted into container 52through lid 54. Thermocouple 60 can be inserted by way of a Swagelok®fitting 62 or other suitable connection. Tubing 64 provides a input flowof a carrier gas into container 52. Tubing 64 preferably includes a shutoff valve 66 and can be inserted through lid 54 by way of a Swagelok®fitting 68 or other suitable connection. Output tube 70 also preferablyincludes a shut off valve 72. Output tube 70 preferably enters intocontainer 52 through lid 54 at a sealed connection 74. Tubes 64 and 70can be made of any suitable inert material such as stainless steel. Asolid precursor can be placed directly within container 52 or it can beplaced within a smaller, open container within container 52.

[0056] Preferred secondary reactants serving as oxygen source include,for example, O₂, CO, CO₂, O₃ and mixtures thereof. The secondaryreactant compound should not react significantly with the zinc precursorprior to entering the reaction zone since this generally would result inthe formation of large particles. If the zinc precursor would react withthe secondary reactant compound, the two reactants can be delivered witha dual nozzle reactant delivery apparatus, as described further below,such that the two reactants do not mix until they are in the reactionchamber. Note that diethyl zinc and O₂ react spontaneously.

[0057] Laser pyrolysis can be performed with a variety of opticalfrequencies. Preferred light sources operate in the infrared portion ofthe electromagnetic spectrum. CO₂ lasers are particularly preferredsources of light. Infrared absorbers for inclusion in the reactantstream include, for example, C₂H₄, isopropyl alcohol, NH₃, SF₆, SiH₄ andO₃. O₃ can act as both an infrared absorber and as an oxygen source. Theradiation absorber, such as the infrared absorber, absorbs energy fromthe radiation beam and distributes the energy to the other reactants todrive the pyrolysis.

[0058] Preferably, the energy absorbed from the light beam increases thetemperature at a tremendous rate, many times the rate that heatgenerally would be produced by exothermic reactions under controlledcondition. While the process generally involves nonequilibriumconditions, the temperature can be described approximately based on theenergy in the absorbing region. The laser pyrolysis process isqualitatively different from the process in a combustion reactor wherean energy source initiates a reaction, but the reaction is driven byenergy given off by an exothermic reaction.

[0059] An inert shielding gas can be used to reduce the amount ofreactant and product molecules contacting the reactant chambercomponents. Appropriate shielding gases include, for example, Ar, He andN₂.

[0060] An appropriate laser pyrolysis apparatus generally includes areaction chamber isolated from the ambient environment. A reactant inletconnected to a reactant delivery apparatus produces a reactant streamthrough the reaction chamber. A laser beam path intersects the reactantstream at a reaction zone. The reactant stream continues after thereaction zone to an outlet, where the reactant stream exits the reactionchamber and passes into a collection apparatus. Generally, the lightsource, such as a laser, is located external to the reaction chamber,and the light beam enters the reaction chamber through an appropriatewindow.

[0061] Referring to FIG. 2, a particular embodiment 100 of a laserpyrolysis system involves a reactant delivery apparatus 102, reactionchamber 104, collection apparatus 106, light source 108 and shieldinggas delivery apparatus 110. Alternative reaction delivery apparatuses102 can be used with the apparatus of FIG. 2. A first reaction deliveryapparatus described below can be used to deliver exclusively gaseousreactants. Two alternative reactant delivery apparatuses are describedfor delivery of one or more reactants as an aerosol.

[0062] Referring to FIG. 3, a first embodiment 112 of reactant deliveryapparatus 102 includes a source 120 of a precursor compound. For liquidor solid reactants, a carrier gas from one or more carrier gas sources122 can be introduced into precursor source 120 to facilitate deliveryof the reactant. Precursor source 120 can be a solid precursor deliveryapparatus 50, as shown in FIG. 1, or other suitable container. Thecarrier gas from carrier gas source 122 preferably is either an infraredabsorber and/or an inert gas. Carrier gas preferably is bubbled througha liquid reactant compound or delivered into a solid reactant deliveryapparatus. The quantity of reactant vapor in the reaction zone isroughly proportional to the flow rate of the carrier gas. A liquid orsolid reactant can be heated to increase its vapor pressure. Similarly,portions of reactant delivery apparatus 102 can be heated to inhibit thedeposition of reactant compound on the walls of the delivery apparatus.

[0063] Alternatively, carrier gas can be supplied directly from infraredabsorber source 124 and/or inert gas source 126, as appropriate. Thegases from precursor source 120 are mixed with gases from infraredabsorber source 124 and/or inert gas source 126 by combining the gasesin a single portion of tubing 128. The gases are combined a sufficientdistance from reaction chamber 104 such that the gases become well mixedprior to their entrance into reaction chamber 104. The combined gas intube 128 passes through a duct 130 into channel 132, which is in fluidcommunication with reactant inlet 134, which can be part of a multipleinlet delivery apparatus, as shown in phantom lines in FIG. 3.

[0064] A second reactant can be supplied from second reactant source138, which can be a liquid reactant delivery apparatus, a solid reactantdelivery apparatus, a gas cylinder or other suitable container orcontainers. If second reactant source 138 delivers a liquid or solidreactant, carrier gas from carrier gas source 122 or an alternativecarrier gas source can be used to facilitate delivery of the reactant.As shown in FIG. 3, second reactant source 138 delivers a secondreactant to duct 130 by way of tube 128. Alternatively, second reactantsource 138 can deliver the second reactant to tube 140 for deliverythrough duct 141 to a second reactant inlet 142, as depicted withphantom lines in FIG. 3. Inlets 134, 142 can be angled slightly towardeach other to facilitate mixing of the gases.

[0065] With alternative delivery through reactant inlets 134 and 142,the first and second reactants are mixed within the reaction chamberafter exiting from the reactant inlets. This is particularlyadvantageous if the reactants spontaneously react, such as diethyl zincand molecular oxygen. If more than two reactants are used, theadditional reactants can similarly be delivered through a singlereactant inlet 134, through two inlets 134, 142, or through more thantwo reactant inlets, as appropriate or desired. Mass flow controllers144 can be used to regulate the flow of gases within the reactantdelivery system of FIG. 3.

[0066] As noted above, the reactant stream can include one or moreaerosols. The aerosols can be formed within reaction chamber 104 oroutside of reaction chamber 104 prior to injection into reaction chamber104. If the aerosols are produced prior to injection into reactionchamber 104, the aerosols can be introduced through reactant inletscomparable to those used for gaseous reactants, such as reactant inlet134 in FIG. 3.

[0067] Referring to FIG. 4, an alternative embodiment 150 of thereactant supply system 102 is used to supply an aerosol to channel 132.As described above, channel 132 forms part of an injection nozzle fordirecting reactants into the reaction chamber and terminates at reactantinlet 134. Reactant supply system 150 includes an aerosol generator 152,carrier gas/vapor supply tube 154 and junction 156. Channel 132, aerosolgenerator 152 and supply tube 154 meet within interior volume 158 ofjunction 156. Supply tube 154 is oriented to direct carrier gas alongchannel 132. Aerosol generator 152 is mounted such that an aerosol 160is generated in the volume of chamber 158 between the opening intochannel 134 and the outlet from supply tube 154.

[0068] Aerosol generator 152 can operate based on a variety ofprinciples. For example, the aerosol can be produced with an ultrasonicnozzle, with an electrostatic spray system, with a pressure-flow orsimplex atomizer, with an effervescent atomizer or with a gas atomizerwhere liquid is forced under significant pressure through a smallorifice and fractured into particles by a colliding gas stream. Suitableultrasonic nozzles can include piezoelectric transducers. Ultrasonicnozzles with piezoelectric transducers and suitable broadband ultrasonicgenerators are available from Sono-Tek Corporation, Milton, NY, such asmodel 8700-120. Suitable aerosol generators are described further incopending and commonly assigned, U.S. patent application Ser. No.09/188,670 to Gardner et al., entitled “REACTANT DELIVERY APPARATUSES,”incorporated herein by reference. Additional aerosol generators can beattached to junction 156 through other ports 162 such that additionalaerosols can be generated in interior 158 for delivery into the reactionchamber.

[0069] Junction 156 includes ports 162 to provide access from outsidejunction 156 to interior 158. Thus, channel 132, aerosol generator 152and supply tube 154 can be mounted appropriately. In one embodiment,junction 156 is cubic with six cylindrical ports 162, with one port 162extending from each face of junction 156. Junction 156 can be made fromstainless steel or other durable, noncorrosive material. A window 161preferably is sealed at one port 162 to provide for visual observationinto interior 158. The port 162 extending from the bottom of junction156preferably includes a drain 163,such that condensed aerosol that isnot delivered through channel 134can be removed from junction 156.

[0070] Carrier gas/vapor supply tube 154is connected to gas source164.Gas source 164can include one or a plurality of gas containers thatare connected to deliver a selected gas or gas mixture to supply tube154.Carrier gas can be passed through a liquid precursor deliveryapparatus or a solid precursor delivery apparatus, such that the carriergas includes vapor of a liquid precursor or a solid precursor. Thus,carrier gas/vapor supply tube 154 can be used to deliver a variety ofdesired gases and/or vapors within the reactant stream including, forexample, laser absorbing gases, reactants, and/or inert gases. The flowof gas from gas source 164 to supply tube 154 preferably is controlledby one or more mass flow controllers 166 or the like. Liquid supply tube168 is connected to aerosol generator 152. Liquid supply tube 168 isconnected to aerosol generator 152 and to liquid supply 170. For theproduction of zinc oxide particles, liquid supply 170 can hold a liquidcomprising a zinc precursor.

[0071] In the embodiment shown in FIG. 4, aerosol generator 152generates an aerosol with momentum roughly orthogonal to the carrier gasflow from tube 154 to channel 132. Thus, carrier gas/vapor from supplytube 154 directs aerosol precursor generated by aerosol generator 152into channel 132. In operation, carrier gas flow directs the aerosoldelivered within chamber 158 into channel 132. In this way, the deliveryvelocity of the aerosol is determined effectively by the flow rate ofthe carrier gas.

[0072] In alternative preferred embodiments, the aerosol generator isplaced at an upward angle relative to the horizontal, such that acomponent of the forward momentum of the aerosol is directed alongchannel 134. In a preferred embodiment, the output directed from theaerosol generator is placed at about a 45° angle relative to the normaldirection defined by the opening into channel 134, i.e. the direction ofthe flow into channel 134 from supply tube 154.

[0073] Referring to FIG. 5, an alternative embodiment 172 of reactantdelivery apparatus 102 is shown for delivery of two aerosol reactants.Aerosol generators 173, 174 deliver aerosol into delivery tubes 132,141, respectively. Channels 132, 141 deliver reactants to reactantinlets 134, 142, respectively. Aerosol generators 173, 174 can operatebased on a variety of principles, as described above with respect toaerosol generator 152.

[0074] Reactant gases, inert gases and/or light absorbing gases can besupplied according to any of a variety of configurations into channels132, 141, as desired, by way of gas sources 176, 177 and gas supplytubes 179, 180. For example, gas supply tubes 179, 180 can connect withchannels 132, 141 a various positions above or below aerosol generators173, 174 and angles relative to the orientation of channels 132, 141.Multiple gas supply tubes can be used for each channel 132, 141, ifdesired. Alternatively, one of aerosol generators 173, 174 can beeliminated such that reactant delivery apparatus 172 delivers an aerosoland a gaseous reactant through reactant inlets 134, 142, respectively.

[0075] Alternative embodiments can be based on variation of theembodiments of FIG. 5 to deliver reactants by way of a single reactantinlet 134. In these embodiments, there is a single channel 132. Thesecond aerosol generator can be eliminated or configured to deliver anaerosol into the same channel as the first aerosol generator. Thus,these alternative embodiments can be used to deliver into reactionchamber 104, an aerosol reactant and a gaseous reactant, two aerosolreactants or more than two reactants with one or more aerosols through asingle reactant inlet 134.

[0076] Referring to FIG. 2, reaction chamber 104 includes a main chamber200. Reactant supply system 102 connects to the main chamber 200 atinjection nozzle 202. Reaction chamber 104 can be heated to keep theprecursor compound in the vapor state. In particular, the entirereaction chamber 104 preferably is heated to about 120° C. when thevapor of a solid precursor is used. Similarly, the argon shielding gaspreferably is heated to about 150° C. when the vapor of a solidprecursor is used. The chamber can be examined for condensation toensure that precursor is not deposited on the chamber.

[0077] The end of injection nozzle 202 has an annular opening 204 forthe passage of inert shielding gas, and reactant inlet 134 for thepassage of reactants to form a reactant stream in the reaction chamber.Reactant inlet 134 preferably is a slit, as shown in the lower leftinsert of FIG. 2. Annular opening 204 has, for example, a diameter ofabout 1.5 inches and a width along the radial direction from about ⅛ into about {fraction (1/16)} in. The flow of shielding gas through annularopening 204 helps to prevent the spread of the reactant gases andproduct particles throughout reaction chamber 104. For embodimentshaving two reactant inlets 134, 142, two slits or other shapes arepresent at the end of nozzle 202, as shown in the insert in the lowerright of FIG. 2.

[0078] Tubular sections 208, 210 are located on either side of injectionnozzle 202. Tubular sections 208, 210 include ZnSe windows 212, 214,respectively. Windows 212, 214 are about 1 inch in diameter. Windows212, 214 are preferably cylindrical lenses with a focal length equal tothe distance between the center of the chamber to the surface of thelens to focus the light beam to a point just below the center of thenozzle opening. Windows 212, 214 preferably have an antireflectivecoating. Appropriate ZnSe lenses are available from Janos Technology,Townshend, Vt. Tubular sections 208, 210 provide for the displacement ofwindows 212, 214 away from main chamber 200 such that windows 212, 214are less likely to be contaminated by reactants and/or products. Window212, 214 are displaced, for example, about 3 cm from the edge of themain chamber 200.

[0079] Windows 212, 214 are sealed with a rubber o-ring to tubularsections 208, 210 to prevent the flow of ambient air into reactionchamber 104. Tubular inlets 216, 218 provide for the flow of shieldinggas into tubular sections 208, 210 to reduce the contamination ofwindows 212, 214. Tubular inlets 216, 218 are connected to inert gassource 220 or to a separate inert gas source. In either case, flow toinlets 216, 218 preferably is controlled by a mass flow controller 222.

[0080] Light source 108 is aligned to generate a light beam 224 thatenters window 212 and exits window 214. Windows 212, 214 define a lightpath through main chamber 200 intersecting the flow of reactants atreaction zone 226. After exiting window 214, light beam 222 strikespower meter 228, which also acts as a beam dump. An appropriate powermeter is available from Coherent Inc., Santa Clara, Calif. Light source108 can be a laser or an intense conventional light source such as anarc lamp. Preferably, light source 108 is an infrared laser, especiallya CW CO₂ laser such as an 1800 watt maximum power output laser availablefrom PRC Corp., Landing, N.J.

[0081] Reactants passing through reactant inlet 134 or inlets 134, 142in injection nozzle 202 initiate a reactant stream. The reactant streampasses through reaction zone 226, where reaction involving the zinc orother metal precursor compound takes place. Heating of the gases inreaction zone 226 is extremely rapid, roughly on the order of 10⁵ degreeC./sec depending on the specific conditions. The reaction is rapidlyquenched upon leaving reaction zone 226, and particles 230 are formed inthe reactant stream. The nonequilibrium nature of the process allows forthe production of nanoparticles with a highly uniform size distributionand structural homogeneity.

[0082] The path of the reactant stream continues to collection nozzle232. Collection nozzle 232 is spaced about 2 cm from injection nozzle202. The small spacing between injection nozzle 202 and collectionnozzle 232 helps reduce the contamination of reaction chamber 104 withreactants and products. Collection nozzle 232 has a circular opening234, as shown in the upper insert of FIG. 2. Circular opening 234 feedsinto collection apparatus 106.

[0083] The chamber pressure is monitored with a pressure gauge 250attached to main chamber 200. The preferred chamber pressure for theproduction of the desired oxides generally ranges from about 80 Torr toabout 700 Torr.

[0084] Reaction chamber 104 has two additional tubular sections notshown. One of the additional tubular sections projects into the plane ofthe sectional view in FIG. 2, and the second additional tubular sectionprojects out of the plane of the sectional view in FIG. 2. When viewedfrom above, the four tubular sections are distributed roughly,symmetrically around the center of the chamber. These additional tubularsections have windows for observing the inside of the chamber. In thisconfiguration of the apparatus, the two additional tubular sections arenot used to facilitate production of particles.

[0085] Collection apparatus 106 preferably includes a curved channel 270leading from collection nozzle 230. Because of the small size of theparticles, the product particles generally follow the flow of the gasaround curves. Collection apparatus 106 includes a filter 272 within thegas flow to collect the product particles. Due to curved section 270,the filter is not supported directly above the chamber. A variety ofmaterials such as Teflon, glass fibers and the like can be used for thefilter as long as the material is inert and has a fine enough mesh totrap the particles. Preferred materials for the filter include, forexample, a glass fiber filter from ACE Glass Inc., Vineland, N.J. andcylindrical Nomex® filters from AF Equipment Co., Sunnyvale, Calif.

[0086] Pump 274 is used to maintain collection system 106 at a selectedpressure. A variety of -different pumps can be used. Appropriate pumpsfor use as pump 274 include, for example, Busch Model B0024 pump fromBusch, Inc., Virginia Beach, Va. with a pumping capacity of about 25cubic feet per minute (cfm) and Leybold Model SV300 pump from LeyboldVacuum Products, Export, Pa. with a pumping capacity of about 195 cfm.It may be desirable to flow the exhaust of the pump through a scrubber276 to remove any remaining reactive chemicals before venting into theatmosphere. The entire system 100 or portions thereof can be placed in afume hood for ventilation purposes and for safety considerations.Generally, light source 108 remains outside of the fume hood because ofits large size.

[0087] Reaction system 100 or components thereof preferably iscontrolled by a computer 280. Generally, computer 280 controls lightsource 108 and monitors the pressure in the reaction chamber 104 by wayof pressure gauge 250 or the like. Computer 280 can be used to controlthe flow of reactants and/or the shielding gas. The pumping rate iscontrolled by either a manual needle valve or an automatic throttlevalve inserted between pump 274 and filter 272. As the chamber pressureincreases due to the accumulation of particles on filter 272, the manualvalve or the throttle valve can be adjusted to maintain the pumping rateand the corresponding chamber pressure.

[0088] The reaction can be continued until sufficient particles arecollected on filter 272 such that pump 274 can no longer maintain thedesired pressure in the reaction chamber 104 against the resistancethrough filter 272. When the pressure in reaction chamber 104 can nolonger be maintained at the desired value, the reaction is stopped, andfilter 272 is removed. With this embodiment, about 1-300 grams ofparticles can be collected in a single run before the chamber pressurecan no longer be maintained. A single run generally can last up to about10 hours depending on the type of particle being produced and the typeof filter being used.

[0089] Referring to FIG. 2, shielding gas delivery apparatus 110includes inert gas source 220 connected to an inert gas duct 292. Inertgas duct 292 flows into annular channel 294. A mass flow controller 296regulates the flow of inert gas into inert gas duct 292. If reactantdelivery apparatus 112 shown in FIG. 3 is used, inert gas source 126 canalso function as the inert gas source for duct 192, if desired.Similarly, separate gas sources can be used to supply inert gas duct 292and tubes 216, 218.

[0090] The reaction conditions can be controlled relatively precisely.The mass flow controllers are quite accurate. The laser generally hasabout 0.5 percent power stability. With either a manual control or athrottle valve, the chamber pressure can be controlled to within about 1percent.

[0091] The configuration of the reactant supply system 102 and thecollection system 106 can be reversed. In this alternativeconfiguration, the reactants are supplied from the top of the reactionchamber, and the product particles are collected from the bottom of thechamber. If mounted below reaction chamber 104, collection system 106may not include a curved section so that the collection filter ismounted directly below reaction chamber 104.

[0092] An alternative design of a laser pyrolysis system has beendescribed in copending and commonly assigned U.S. patent applicationSer. No. 08/808,850, entitled “Efficient Production of Particles byChemical Reaction,” incorporated herein by reference. This alternativedesign is intended to facilitate production of commercial quantities ofparticles by laser pyrolysis. The reaction chamber is elongated alongthe laser beam in a dimension perpendicular to the reactant stream toprovide for an increase in the throughput of reactants and products. Theoriginal design of the apparatus was based on the introduction ofgaseous reactants. Alternative embodiments for the introduction of anaerosol into an elongated reaction chamber are described in copendingand commonly assigned U.S. patent application Ser. No. 09/188,670 toGardner et al., filed on Nov. 9, 1998, entitled “Reactant DeliveryApparatuses,” incorporated herein by reference.

[0093] In general, the alternative pyrolysis apparatus includes areaction chamber designed to reduce contamination of the chamber walls,to increase the production capacity and to make efficient use ofresources. To accomplish these objectives, an elongated reaction chamberis used that provides for an increased throughput of reactants andproducts without a corresponding increase in the dead volume of thechamber. The dead volume of the chamber can become contaminated withunreacted compounds and/or reaction products.

[0094] The design of the improved reaction chamber 300 is shownschematically in FIG. 6. A reactant inlet 302 leads to main chamber 304.Reactant inlet 302 conforms generally to the shape of main chamber 304.Main chamber 304 includes an outlet 306 along the reactant/productstream for removal of particulate products, any unreacted gases andinert gases. Shielding gas inlets 310 are located on both sides ofreactant inlet 302. Shielding gas inlets are used to form a blanket ofinert gases on the sides of the reactant stream to inhibit contactbetween the chamber walls and the reactants or products.

[0095] Tubular sections 320, 322 extend from the main chamber 304.Tubular sections 320, 322 hold windows 324, 326 to define a light beampath 328 through the reaction chamber 300. Tubular sections 320, 322 caninclude inert gas inlets 330, 332 for the introduction of inert gas intotubular sections 320, 322.

[0096] The improved reaction system includes a collection apparatus toremove the nanoparticles from the reactant stream. The collection systemcan be designed to collect particles in a batch mode with the collectionof a large quantity of particles prior to terminating production.Alternatively, the collection system can be designed to run in acontinuous production mode by switching between different particlecollectors within the collection apparatus or by providing for removalof particles without exposing the collection system to the ambientatmosphere. An alternative preferred embodiment of a collectionapparatus for continuous particle production is described in copendingand commonly assigned U.S. patent application Ser. No. 09/107,729 toGardner et al., entitled “Particle Collection Apparatus And AssociatedMethods,” incorporated herein by reference. The collection apparatus caninclude curved components within the flow path similar to curved portionof the collection apparatus shown in FIG. 2.

[0097] Referring to FIG. 7, a specific embodiment 350 of a laserpyrolysis reaction system with an elongated reaction chamber is shown.In this embodiment, the reaction chamber can be used with a reactiondelivery apparatus designed for the delivery of only gaseous reactantsor with an reactant delivery apparatus that can deliver aerosolreactants along with any desired gases. Laser pyrolysis reactionapparatus 350 includes reaction chamber 352, a particle collectionapparatus 354, light source 356 and a reactant delivery system attachedat inlet 364.

[0098] Reaction chamber 352 includes inlet 364 at the bottom of reactionchamber 352 where the reactant delivery apparatus connects with reactionchamber 352. Nozzles associated with the reactant delivery apparatus canextend into reaction chamber 352. Gaseous reactants can be deliveredthrough a nozzle elongated to conform generally to the elongation ofreaction chamber 352. Similarly, aerosols can be delivered to accountfor the elongated shape of the reaction chamber.

[0099] In this embodiment, the reactants are delivered from the bottomof reaction chamber 352 while the products are collected from the top ofreaction chamber 352. The configuration can be reversed with thereactants supplied from the top and product collected from the bottom,if desired. Shielding gas conduits can be located in appropriatepositions around reactant inlet 364. The shielding gas conduits directshielding gas along the walls of reaction chamber 352 to inhibitassociation of reactant gases or products with the walls.

[0100] Reaction chamber 352 is elongated along one dimension denoted inFIG. 7 by “w”. A laser beam path 366 enters reaction chamber 352 througha window 368 displaced along a tube 370 from main chamber 372 andtraverses the elongated direction of reaction chamber 352. The laserbeam passes through tube 374 and exits window 376. In one preferredembodiment, tubes 370 and 374 displace windows 368 and 376 about 11inches from main chamber 372. The laser beam terminates at beam dump378. In operation, the laser beam intersects a reactant stream generatedthrough reactant inlet 364.

[0101] The top of main chamber 372 opens into particle collection system354. Particle collection system 354 includes outlet duct 380 connectedto the top of main chamber 372 to receive the flow from main chamber372. Outlet duct 380 carries the product particles out of the plane ofthe reactant stream to a cylindrical filter 382, as shown in FIG. 8.Filter 382 has a cap 384 on one end to block direct flow into the centerof filter 382. The other end of filter 382 is fastened to disc 386. Vent388 is secured to the center of disc 386 to provide access to the centerof filter 382. Vent 388 is attached by way of ducts to a pump.

[0102] Thus, product particles are trapped on filter 382 by the flowfrom the reaction chamber 352 to the pump. Suitable pumps were describedabove with respect to the first laser pyrolysis apparatus in FIG. 2.Suitable filters for use as filter 382 include, for example, an aircleaner filter for a Saab 9000 automobile (Purilator part A44-67), whichis wax impregnated paper with Plasticol™ or polyurethane end cap 384.The configuration of the reactant injection components and thecollection system can be reversed such that the particles are collectedat the top of the apparatus.

[0103] The collection apparatus shown in FIGS. 7 and 8 is suitable forthe operation of reaction chamber 352 in batch mode, where operation isstopped when filter 382 can no longer collect additional particles.Alternative collection apparatuses are suitable for operating reactionchamber 352 in continuous operation, as described above.

[0104] The dimensions of elongated reaction chamber 352 and reactantinlet 364 preferably are designed for high efficiency particleproduction. Reasonable dimensions for reactant inlet 364 for theproduction of zinc oxide nanoparticle, when used with a 1800 watt CO₂laser, are from about 5 mm to about 1 meter.

[0105] As noted above, properties of the zinc oxide particles can bemodified by further processing. In particular, zinc oxide nanoscaleparticles can be heated in an oven in an oxidizing environment or aninert environment to alter the oxygen content, to change the crystallattice, or to remove adsorbed compounds on the particles to improve thequality of the particles.

[0106] The starting material for the heat treatment can be anycollection of zinc oxide particles of any size and shape. Nanoscaleparticles are preferred starting materials. Suitable nanoscale startingmaterials have been produced by laser pyrolysis. In addition, particlesused as starting material can have been subjected to one or more priorheating steps under different conditions. The processing of metal oxidenanoscale particles in an oven is discussed further in copending andcommonly assigned, U.S. patent application Ser. No. 08/897,903, filedJul. 21, 1997, entitled “Processing of Vanadium Oxide Particles WithHeat,” incorporated herein by reference.

[0107] The zinc oxide particles are preferably heated in an oven or thelike to provide generally uniform heating. The processing conditionsgenerally are mild, such that significant amounts of particle sinteringdoes not occur. The temperature of heating preferably is low relative tothe melting point of both the starting material and the productmaterial.

[0108] The atmosphere for the heating process can be an oxidizingatmosphere or an inert atmosphere, although a different product may formdepending on the nature of the gas in contact with the sample. Theatmosphere over the particles can be static, or gases can be flowedthrough the system. Appropriate oxidizing gases include, for example,O₂, O₃, CO, CO₂, and combinations thereof. The O₂ can be supplied asair.

[0109] Oxidizing gases optionally can be mixed with inert gases such asAr, He and N₂. When inert gas is mixed with the oxidizing gas, the gasmixture can be from about 1 percent oxidizing gas to about 99 percentoxidizing gas, and more preferably from about 5 percent oxidizing gas toabout 99 percent oxidizing gas. Alternatively, either essentially pureoxidizing gas or pure inert gas can be used, as desired.

[0110] The precise conditions can be altered to vary the characteristicsof the zinc oxide product produced. For example, the temperature, timeof heating, heating and cooling rates, the gases and the exposureconditions with respect to the gases can all be changed, as desired.Generally, while heating under an oxidizing atmosphere, the longer theheating period the more oxygen that is incorporated into the material,prior to reaching equilibrium. Once equilibrium conditions are reached,the overall conditions determine the crystalline phase of the powders.

[0111] A variety of ovens or the like can be used to perform theheating. An example of a suitable apparatus 400 to perform thisprocessing is displayed in FIG. 6. Apparatus 400 includes a jar 402,which can be made from glass or other inert material, into which theparticles are placed. Suitable glass jars are available from Ace Glass(Vineland, N.J.). The top of glass jar 402 is sealed to a glass cap 404,with a Teflon® gasket 405 between jar 402 and cap 404. Cap 404 can beheld in place with one or more clamps. Cap 404 includes a plurality ofports 406, each with a Teflon® brushing. A multiblade stainless steelstirrer 408 preferably is inserted through a central port 406 in cap404. Stirrer 408 is connected to a suitable motor.

[0112] One or more tubes 410 are inserted through ports 406 for thedelivery of gases into jar 402. Tubes 410 can be made from stainlesssteel or other inert material. Diffusers 412 can be included at the tipsof tubes 410 to disburse the gas within jar 402. A heater/furnace 414generally is placed around jar 402. Suitable resistance heaters areavailable from Glas-col (Terre Haute, Ind.). On port preferably includesa T-connection 416. The temperature within jar 402 can be measured witha thermocouple 416 inserted through a T-connection 416. T-connection 416can be further connected to a vent 418. Vent 418 provides for theventing of gas circulating through jar 402. Preferably, vent 418 isvented to a fume hood or alternative ventilation equipment.

[0113] Preferably, desired gases are flowed through jar 402. Tubes 410generally are connected to an oxidizing gas source 420 and/or an inertgas source 422. Oxidizing gas source 420 and inert gas source 422 can begas cylinders or any other suitable containers. Oxidizing gas, inert gasor a combination thereof to produce the desired atmosphere are placedwithin jar 402 from the appropriate gas source(s).

[0114] Various flow rates can be used. The flow rate preferably isbetween about 1 standard cubic centimeters per minute (sccm) to about1000 sccm and more preferably from about 10 sccm to about 500 sccm. Theflow rate generally is constant through the processing step, althoughthe flow rate and the composition of the gas can be variedsystematically over time during processing, if desired. Alternatively, astatic gas atmosphere can be used.

[0115] For the processing of zinc oxide, for example, the temperaturespreferably range from about 50° C. to about 500° C. and more preferablyfrom about 60° C. to about 400° C. The particles preferably are heatedfor greater than about 5 minutes, and generally, for about 5 minutes toabout 100 hours. The heating preferably is continued for from about 2hours to about 100 hours, more preferably from about 2 hours to about 50hours. For certain target product particles, additional heating does notlead to further variation in the particle composition.

[0116] Some empirical adjustment may be required to produce theconditions appropriate for yielding a desired material. The use of mildconditions avoids interparticle sintering resulting in larger particlesizes. Some controlled sintering of the particles can be performed atsomewhat higher temperatures to produce slightly larger, averageparticle diameters.

[0117] B. Particle Properties

[0118] A collection of particles of interest generally has an averagediameter for the primary particles of less than about 150 nm, preferablyless than about 95 nm, more preferably from about 5 nm to about 50 nm,and even more preferably from about 5 nm to about 25 nm. The primaryparticles usually have a roughly spherical gross appearance. Upon closerexamination, crystalline zinc oxide particles generally have facetscorresponding to the underlying crystal lattice. Nevertheless, theprimary particles tend to exhibit growth that is roughly equal in thethree physical dimensions to give a gross spherical appearance.Generally, 95 percent of the primary particles, and preferably 99percent, have ratios of the dimension along the major axis to thedimension along the minor axis less than about 2. Diameter measurementson particles with asymmetries are based on an average of lengthmeasurements along the principle axes of the particle.

[0119] Because of their small size, the primary particles tend to formloose agglomerates due to van der Waals and other electromagnetic forcesbetween nearby particles. Nevertheless, the nanometer scale of theprimary particles is clearly observable in transmission electronmicrographs of the particles. The particles generally have a surfacearea corresponding to particles on a nanometer scale as observed in themicrographs. Furthermore, the particles can manifest unique propertiesdue to their small size and large surface area per weight of material.

[0120] Furthermore, the particles manifest unique properties due totheir small size and large surface area per weight of material. ofparticular relevance, the particles have an altered band structure, asdescribed further below. The high surface area generally leads to highluminosity of the particles.

[0121] The primary particles preferably have a high degree of uniformityin size. As determined from examination of transmission electronmicrographs, the primary particles generally have a distribution insizes such that at least about 95 percent, and preferably 99 percent, ofthe primary particles have a diameter greater than about 40 percent ofthe average diameter and less than about 160 percent of the averagediameter. Preferably, the primary particles have a distribution ofdiameters such that at least about 95 percent of the primary particleshave a diameter greater than about 60 percent of the average diameterand less than about 140 percent of the average diameter.

[0122] Furthermore, essentially no primary particles have an averagediameter greater than about 4 times the average diameter and preferably3 times the average diameter, and more preferably 2 times the averagediameter. In other words, the particle size distribution effectivelydoes not have a tail indicative of a small number of particles withsignificantly larger sizes. This is a result of the small reactionregion and corresponding rapid quench of the particles. An effective cutoff in the tail indicates that there are less than about 1 particle in10⁶ have a diameter greater than a particular cut off value above theaverage diameter. The narrow size distributions, lack of a tail in thedistributions and the roughly spherical morphology can be exploited in avariety of applications, especially for phosphors with narrow emissionranges.

[0123] At small crystalline diameters the band properties of theparticles are altered. The increase in band gap is approximately inproportion to 1/(particle size)². For especially small particle sizes,the density of states may become low enough that the band descriptionmay become incomplete as individual molecular orbitals may play a moreprominent role. The qualitative trends should hold regardless of theneed to account for a molecular orbital description of the electronicproperties.

[0124] In addition, with a uniform distribution of small particles, theemission spectrum narrows because of the reduction of inhomogeneousbroadening. The result is a sharper emission spectrum with an emissionmaximum that depends on the average particle diameter. Thus, the use ofvery small particles may allow. for adjustment of emissioncharacteristics without the need to activate the phosphorescentparticles with a second metal.

[0125] Furthermore, the small size of the particles allows for theformation of very thin layers. Thin layers are advantageous for use withlow velocity electrons since the electrons may not penetrate deeplywithin a layer. The small size of the particles is also conducive to theformation of small patterns, for example using photolithography, withsharp edges between the elements of the pattern. The production ofsmall, sharply separated elements is important for the formation of highresolution displays.

[0126] In addition, the nanoparticles generally have a very high puritylevel. The crystalline zinc oxide nanoparticles produced by the abovedescribed methods are expected to have a purity greater than thereactant gases because the crystal formation process tends to excludecontaminants from the lattice. Furthermore, crystalline zinc oxideparticles produced by laser pyrolysis generally have a high degree ofcrystallinity and few surface distortions.

[0127] Although under certain conditions mixed phase material may beformed, laser pyrolysis generally can be effectively used to producesingle phase crystalline particles. Primary particles generally consistof single crystals of the material. The single phase, single crystalproperties of the particles can be used advantageously along with theuniformity and narrow size distribution. Under certain conditions,amorphous particles may be formed by laser pyrolysis. Some amorphousparticles can be heat treated under mild conditions to form crystallineparticles.

[0128] Zinc oxides can have a stoichiometry of, at least, ZnO (hexagonalcrystal, Zincite structure) or ZnO₂. The production parameters can bevaried to select for a particular stoichiometry of zinc oxide.

[0129] C. Phosphors, Displays and Resistors

[0130] The particles described in this application can be used asphosphors. The phosphors emit light, preferably visible light, followingexcitation. A variety of ways can be used to excite the phosphors, andparticular phosphors may be responsive to one or more of the excitationapproaches. Particular types of luminescence includecathodoluminescence, photoluminescence and electroluminescence which,respectively, involve excitation by electrons, light and electricfields. Many materials that are suitable as cathodoluminescencephosphors are also suitable as electroluminescence phosphors.

[0131] In particular, the particles preferably are suitable forlow-velocity electron excitation, with electrons accelerated withpotentials below 1 KV, and more preferably below 100 V. The small sizeof the particles makes them suitable for low velocity electronexcitation. Furthermore, preferred particles with average diameters lessthan about 100 nm can produce high luminescence with low electronvelocity excitation. The phosphor particles can be used to produce anyof a variety of display devices based on low velocity electrons, highvelocity electrons, or electric fields.

[0132] Referring to FIG. 10, a display device 500 includes an anode 502with a phosphor layer 504 on one side. The phosphor layer faces anappropriately shaped cathode 506, which is the source of electrons usedto excite the phosphor. A grid cathode 508 can be placed between theanode 502 and the cathode 506 to control the flow of electrons from thecathode 506 to the anode 502.

[0133] Cathode ray tubes (CRTs) have been used for a long time forproducing images. CRTs generally use relatively higher electronvelocities. Phosphor particles, as described above, can still be usedadvantageously as a convenient way of supplying particles of differentcolors, reducing the phosphor layer thickness and decreasing thequantity of phosphor for a given luminosity. CRTs have the generalstructure as shown in FIG. 10, except that the anode and cathode areseparated by a relatively larger distance and steering electrodes ratherthan a grid electrode generally are used to guide the electrons from thecathode to the anode.

[0134] Other preferred applications include the production of flat paneldisplays. Flat panel displays can be based on, for example, liquidcrystals or field emission devices. Liquid crystal displays can be basedon any of a variety of light sources. Phosphors can be useful in theproduction of lighting for liquid crystal displays. Referring to FIG.11, a liquid crystal element 530 includes at least partially lighttransparent substrates 532, 534 surrounding a liquid crystal layer 536.Lighting is provided by a phosphor layer 538 on an anode 540. Cathode542 provides a source of electrons to excite the phosphor layer 538.Alternative embodiments are described, for example, in U.S. Pat. No.5,504,599, incorporated herein by reference.

[0135] Liquid crystal displays can also be illuminated with backlightingfrom an electroluminescent display. Referring to FIG. 12,electroluminescent display 550 has a conductive substrate 552 thatfunctions as a first electrode. Conductive substrate 552 can be madefrom, for example, aluminum, graphite or the like. A second electrode554 is transparent and can be formed from, for example, indium tinoxide. A dielectric layer 556 may be located between electrodes 552,554, adjacent to first electrode 552. Dielectric layer 556 includes adielectric binder 558 such as cyanoethyl cellulose or cyanoethyl starch.Dielectric layer 556 can also include ferroelectric material 560 such asbarium titanate. Dielectric layer 556 may not be needed for dc-driven(in contrast with ac-driven) electro-luminescent devices. A phosphorlayer 562 is located between transparent electrode 554 and dielectriclayer 556. Phosphor layer 562 includes electro-luminescent particles564, such as zinc oxide nanoparticles, in a dielectric binder 566. Therelative sizes of the materials are not necessarily to scale.

[0136] Electroluminescent display 550 also can be used for other displayapplications such as automotive dashboard and control switchillumination. In addition, a combined liquid crystal/electroluminescentdisplay has been designed. See, Fuh, et al., Japan J. Applied Phys.33:L870-L872 (1994), incorporated herein by reference.

[0137] Referring to FIG. 13, a display 580 based on field emissiondevices involves anodes 582 and cathodes 584 spaced a relatively smalldistance apart. Each electrode pair form an individually addressablepixel. A phosphor layer 586 is located between each anode 582 andcathode 584. The phosphor layer 586 includes phosphorescentnanoparticles as described above. Phosphorescent particles with aselected emission frequency can be located at a particular addressablelocation. The phosphor layer 586 is excited by low velocity electronstravelling from the cathode 584 to the anode 582. Grid electrodes 588can be used to accelerate and focus the electron beam as well as act asan on/off switch for electrons directed at the phosphor layer 586. Anelectrically insulating layer is located between anodes 582 and gridelectrodes 588. The elements are generally produced by photolithographyor a comparable techniques such as sputtering and chemical vapordeposition for the production of integrated circuits. As shown in FIG.13, anodes 582 should be at least partially transparent to permittransmission of light emitted by phosphor 586.

[0138] Alternatively, U.S. Pat. No. 5,651,712, incorporated herein byreference, discloses a display incorporating field emission deviceshaving a phosphor layer oriented with an edge (rather than a face) alongthe desired direction for light propagation. The construction displayedin this patent incorporates color filters to produce a desired coloremission rather than using phosphors that emit at desired frequencies.Based on the particles described above, selected phosphor particlespreferably would be used to produce the different colors of light,thereby eliminating the need for color filters.

[0139] The nanoparticles can be directly applied to a substrate toproduce the above structures. Alternatively, the nanoparticles can bemixed with a binder such as a curable polymer for application to asubstrate. The composition involving the curable binder and the phosphornanoparticles can be applied to a substrate by photolithography or othersuitable technique for patterning a substrate such as used in theformation of integrated circuit boards. Once the composition isdeposited at a suitable positions on the substrate, the material can beexposed to suitable conditions to cure the polymer. The polymer can becurable by electron beam radiation, UV radiation or other suitabletechniques.

[0140] Zinc oxide particles can also be used in the production ofelectrical components, such as resistors. A cross section of a resistoris shown schematically in FIG. 14. Resistor 480 include metal leads 482,484. Zinc oxide nanoparticles 486 are embedded within a composite 488.The composite can include additional metal oxide particles and/orsilicon carbide particles. Resistor 480 includes an electricallyinsulating cover 490.

[0141] In particular, zinc oxide powders can be uses for the productionof varisters, which have a resistance that varies with applied voltage.The resistance material can further include various additionaladditives, such as Bi₂O₃, Sb₂O₃, SiO₂, Co₂O₃, MnO₂, other metal oxidesand SiC. The various materials are combined with a binder. The resultingmaterial can be calcined. Appropriate leads and insulating coatings canbe attached to the structure to form an electrical component. Theformation of varisters is described further in U.S. Pat. No. 5,250,281to Imai et al, incorporated herein by reference.

[0142] The phosphor particles can be adapted for use in a variety ofother devices beyond the representative embodiments specificallydescribed.

EXAMPLE

[0143] The synthesis of zinc oxide particles described in this examplewas performed by laser pyrolysis. The particles were produced usingessentially the laser pyrolysis apparatus of FIG. 2, described above,using the reactant delivery apparatus of FIG. 4.

[0144] The zinc nitrate.6H₂O (Aldrich Chemical Co., Milwaukee, Wis.)precursor was carried into the reaction chamber as an aerosol of a 4Maqueous zinc nitrate solution made with deionized water. C₂H₄ gas wasused as a laser absorbing gas, molecular oxygen was used as an oxygensource, and Argon was used as an inert gas. The Ar, O₂ and C₂H₄ weresupplied as carrier gases in the apparatus of FIG. 4. The reactantmixture containing Zn(NO₃)₂, Ar, H₂O, O₂ and C₂H₄ was introduced intothe reactant nozzle for injection into the reaction chamber. Thereactant nozzle had an opening with dimensions of ⅝ in.×¼ in. Additionalparameters of the laser pyrolysis synthesis relating to the particles ofthe Example are specified in Table 1. TABLE 1 Zinc Oxide (ZnO) +Crystalline Phase unidentified Crystal Structure Zincite Pressure (Torr)450 Argon F.R.-Window (SLM) 2.24 Argon F.R.-Shielding (SLM) 9.86Ethylene (SLM) 1.42 Argon (SLM) 8.35 Oxygen (SLM) 1.71 Laser Input(Watts) 970 Laser Output (Watts) 770 Precursor Zinc Nitrate solution inwater Precursor Molarity 4 M Precursor Temperature ° C. Room Temperature

[0145] The production rate of zinc oxide particles was about 3 g/hr. Toevaluate the atomic arrangement, the samples were examined by x-raydiffraction using the Cu(Kα) radiation line on a Siemens D500 x-raydiffractometer. X-ray diffractograms for a sample produced under theconditions specified in Table 1 is shown in FIG. 15. The particles hadan x-ray diffractogram corresponding to zinc oxide, ZnO. The sharp peakin the diffractogram at a value of 2θ equal to about 22° wasunidentified, indicating that another crystalline phase was present inthe sample. Also, a broad peak centered at a value of 2θ equal to about18° indicates the presence of an unidentified amorphous phase, possiblyamorphous zinc oxide. Thus, three phases of materials evidently werepresent in the product powders.

[0146] An elemental analysis of the product powders yielded 71.55percent by weight zinc and minor contaminants of 1.68 percent carbon,0.2 percent nitrogen and 0.08 percent hydrogen. The particles had a graycolor presumably due to the presence of the carbon. Assuming that theremaining weight is oxygen, the material is somewhat rich in oxygenrelative to ZnO. Previously unknown phases of zinc oxide may be present.The carbon contamination can be removed by heating under mild conditionsin an oxygen atmosphere. The removal of carbon contaminants from metaloxide nanoparticles is described further in copending and commonlyassigned U.S patent application Ser. No. 09/136,483 to Kumar et al.,entitled “Aluminum Oxide Particles,” incorporated herein by reference.

[0147] Based on these results, the reaction conditions can be variedempirically to obtain single phase crystalline ZnO by varying theparameters, such as reactant flow rates, pressure and laserpower/temperature, to locate the conditions for the production of singlephase zinc oxide. Since significant quantities of crystalline ZnO wereproduced under the conditions presented in Table 1, parameters suitablefor production of the single phase material will be similar to theseparameters.

[0148] Transmission electron microscopy (TEM) was used to determineparticle sizes and morphology. A TEM micrograph for the particlesproduced under the conditions of Table 1 is displayed in FIG. 16. Thecorresponding particle size distribution is shown in FIG. 17. Theapproximate size distribution was determined by manually measuringdiameters of the particles distinctly visible in the micrograph of FIG.16. Only those particles having clear particle boundaries were measuredto avoid regions distorted or out of focus in the micrograph.Measurements so obtained should be more accurate and are not biasedsince a single view cannot show a clear view of all particles. Theparticle size distribution shown in FIG. 17 has a bimodal or trimodaldistribution indicative of multiple phases of materials. As noted above,different phases of materials form different size particles. If thelaser pyrolysis is performed under conditions selected to yield a singlephase of product particles, a narrow size distribution should result forparticles of the particular phase. In particular, the resulting singlephase crystalline ZnO would have an extremely narrow particle sizedistribution corresponding roughly to one of the three peak widths fromFIG. 17.

[0149] The embodiments described above are intended to be illustrativeand not limiting. Additional embodiments are within the claims below.Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A collection of particles comprising crystallinezinc oxide, the collection of particles having an average diameter lessthan about 95 nm.
 2. The collection of particles of claim 1 wherein thecollection of particles have an average diameter from about 5 nm toabout 50 nm.
 3. The collection of particles of claim 1 wherein thecollection of particles have an average diameter from about 5 nm toabout 25 nm.
 4. The collection of particles of claim 1 whereineffectively no particles have a diameter greater than about four timesthe average diameter of the collection of particles.
 5. The collectionof particles of claim 1 wherein effectively no particles have a diametergreater than about three times the average diameter of the collection ofparticles.
 6. The collection of particles of claim 1 wherein thecollection of particles have a distribution of particle sizes such thatat least about 95 percent of the particles have a diameter greater thanabout 40 percent of the average diameter and less than about 160 percentof the average diameter.
 7. An electrical resistor component comprisingthe collection of particle of claim
 1. 8. The electrical resistorcomponent of claim 7 wherein the component is a varister.
 9. Theelectrical resistor component of claim 8 wherein the varister has anon-linear voltage dependance.
 10. A method for producing zinc oxideparticles, the method comprising pyrolyzing a reactant stream comprisinga zinc precursor and an oxygen source in a reaction chamber, where thepyrolysis is driven by heat absorbed from a light beam.
 11. The methodof claim 10 wherein the zinc oxide particles have an average diameterless than about 150 nm.
 12. The method of claim 10 wherein the zincoxide particles have an average diameter from about 5 nm to about 50 nm.13. The method of claim 10 wherein the light beam is produced by a CO₂laser.
 14. The method of claim 10 wherein the zinc precursor comprises azinc compound selected from the group consisting of ZnCl₂ and Zn(NO₃)₂.15. The method of claim 10 wherein the zinc precursor within thereactant stream is a vapor.
 16. A method for producing zinc oxideparticles, the method comprising pyrolyzing a reactant stream comprisinga zinc precursor aerosol in a reaction chamber, where the pyrolysis isdriven by heat absorbed from a light beam.
 17. The method of claim 16wherein the zinc precursor comprises a compound selected from the groupconsisting of zinc chloride, zinc nitrate, dimethyl zinc and diethylzinc.
 18. The method of claim 16 wherein the reactant stream furthercomprises an oxygen source.
 19. The method of claim 17 wherein the lightbeam is produced with a CO₂ laser.
 20. The method of claim 16 where inthe zinc precursor comprises diethyl zinc and the reactant streamfurther comprises an oxygen source, the diethyl zinc and the oxygensource being delivered into the reaction chamber by way of separatenozzles.
 21. A reaction system comprising: a reaction chamber having anoutlet along a reactant path; a reactant delivery apparatus thatcombines reactants within the reaction chamber from a plurality ofreactant inlets, such that the combined reactants are directed along thereactant path; and a light source that directs a light beam at thecombined reactants along the reactant path.
 19. The reaction system ofclaim 18 wherein the reactant delivery apparatus comprises: an aerosoldelivery apparatus that produces an aerosol along the reactant path; anda gaseous reactant delivery system that combines a gaseous reactant withthe aerosol along the reactant path within the reaction chamber.
 20. Thereactant system of claim 19 wherein the aerosol delivery apparatuscomprises a conduit connected to a gas supply.
 21. The reaction systemof claim 18 wherein the reactant delivery apparatus comprises two gasports oriented to combine two gaseous reactants along the reactant pathwithin the reaction chamber.
 22. The reaction system of claim 18 whereinthe reactant delivery apparatus comprises two aerosol deliveryapparatuses oriented to combine two aerosol reactants along the reactantpath within the reaction chamber.
 23. The reaction system of claim 22wherein at least one of the aerosol delivery apparatuses comprise aconduit connected to a gas supply.
 24. The reaction system of claim 18further comprising a shielding gas port oriented to direct a shieldinggas to limit the spread of the combined reactants along the reactantpath.
 25. The reactant system of claim 18 wherein the reactant deliveryapparatus combines three reactants within the reaction chamber along thereactant path.